EP2013069B1 - Method and system for determining an optimal steering angle in understeer situations in a vehicle - Google Patents

Description

The invention relates to a method for determining an optimal steering angle in understeer situations of a vehicle.

The invention also relates to a device for determining an optimal steering angle in understeer situations of a vehicle.

Modern vehicles use electronically controllable motors in the steering line on the one hand for selectively influencing the steering torque to be applied by the driver (power steering) and on the other hand for the targeted adjustment of steering angles independent of the driver (overlay steering). In addition to these steering systems, which act on the front axle of the vehicle, use modern chassis regulations such. Global Chassis Control (GCC) also rear-axle steering to control the driving dynamics.

In order to influence the steering torque to be applied by the driver, various control and / or control structures, each oriented to the specific driving situation, are known. For example, in the case of oversteering driving situations, a control based on a yaw rate reference ( WO 2005/054039 A1 ) and brakes on μ-split use a control based on ABS wheel information ( WO 2005/054040 A1 ). Understeering driving situations, the present in recognizing the situation steering angle is "frozen", ie by a Torque control should be given to the driver the recommendation not to increase the steering angle and thereby aggravate the situation in the sequence. The disadvantage of this concept is that the driver is given no feedback about the maximum possible lateral force.

It would therefore be desirable if the driver could be assisted to set a maximum lateral force on the wheels.

From the DE 10 2005 036 708 A1 Stabilizing means are known which actuate the steering means in response to a Seitenkraftbeiwert at least one of the steered wheels for adjusting a steering angle stabilizing the vehicle, wherein the stabilizing means adjust a slip angle of the steered wheels such that the Seitenkraftbeiwert does not exceed the range of the maximum substantially.

In DE 100 39 782 A1 a method according to the preamble of claim 1 is known.

The invention has for its object to improve a method of the type mentioned so that the driver is reliably supported during an understeering driving situation in the stabilization of the vehicle.

This object is achieved by a method according to claim 1 and by a device according to claim 8.

The invention provides a method for determining an optimum steering angle in understeer situations of a vehicle, in which a first component is taken into account in the determination, which reproduces the adhesion coefficient in the transverse direction, in which a second component is used, which reflects a kinematics component and in which third Part is taken into account, which reflects the float angle and in which the steering angle δ _{V , lim is} determined by adding the coefficient of adhesion, the kinematics part and the slip angle.

The kinematics component is the proportional velocity from the vehicle rotation relative to the center of gravity velocity.

The system for controlling electronically controllable motors in the steering line allow the driver to advantageously set the side force maximum in understeer situations by power steering. This assistance in steering can stabilize the vehicle in critical driving situations. This all-wheel steering are considered.

Advantageously, the slip angle during understeer can be estimated according to the relationship β≈0, since the slip angle at the beginning of the understeering driving situation is approximately zero.

oder mit der Kraftschlussausnutzung im Fahrzeugschwerpunkt $$\mathrm{punkt}{\mathrm{\mu }}_{\mathit{CoG}}=\frac{\sqrt{a_x^2+a_y^2}}{g},$$

oder mit der Kraftschlussausnutzung an der Hinterachse $${\mathrm{\mu }}_{\mathit{HA}}=\frac{\sqrt{{\left(a_x+l_H{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_H\ddot{\mathrm{\psi }}\right)}^2}}{g}\mathrm{.}$$

ermittelt.Advantageously, it is determined road friction coefficient at the axles and the vehicle's center of gravity. The road _{friction coefficient} μ ₀ = max (μ _VA , μ _CoG , μ _HA ) becomes at least one of the relationships, with the adhesion utilization for the front axle $${\mathrm{\mu }}_{\mathit{VA}}=\frac{\sqrt{{\left(a_x-l_V{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_V\ddot{\mathrm{\psi }}\right)}^2}}{G}.$$

or with the adhesion utilization in the vehicle's center of gravity $$\mathrm{Point}{\mathrm{\mu }}_{\mathit{CoG}}=\frac{\sqrt{a_x^2+a_y^2}}{G}.$$

or with the traction on the rear axle $${\mathrm{\mu }}_{\mathit{HA}}=\frac{\sqrt{{\left(a_x+l_H{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_H\ddot{\mathrm{\psi }}\right)}^2}}{G}\mathrm{,}$$

determined.

ermittelt wird.Advantageously, the optimum steering angle is calculated in a model in which the steering angle in terms of magnitude according to the relationship $${\mathrm{\delta }}_{V.\lim }=\frac{l_V}{v_x}\left|\dot{\mathrm{\psi }}\right|+\frac{2}{C_{\mathrm{\alpha }0}}{\hat{\mathrm{\mu }}}_0$$

is determined.

The steering angle δ _{V , lim} or a steering angle δ _{V , lim} multiplied by a factor k is used as a target value for a steering angle control or a steering torque control.

aktiviert wird bzw. nach der Beziehung $${\mathbf{\delta }}_{\mathrm{V},\lim }>\left|{\mathbf{\delta }}_{\mathrm{V}}\right|$$

deaktiviert wird.Furthermore, it is advantageously provided that a steering torque control according to the relationship $${\mathbf{\delta }}_{\mathrm{V}.\lim }<\left|{\mathbf{\delta }}_{\mathrm{V}}\right|$$

is activated or after the relationship $${\mathbf{\delta }}_{\mathrm{V}.\lim }>\left|{\mathbf{\delta }}_{\mathrm{V}}\right|$$

is deactivated.

The invention also provides an advantageous apparatus for carrying out the method according to the invention.

The device for determining an optimum steering angle in understeer situations of a vehicle is based on a determination unit for determining a stabilizing steering angle taking into account a model-based adhesion coefficient component, a model-based kinematics component and a slip angle.

Further advantages and expedient developments of the invention will become apparent from the dependent claims and the following description of preferred embodiments with reference to FIGS.

Fig. 1

ein Blockschaltbild mit einer Übersicht eines Regelsystems einer elektrischen Servolenkung zur Ermittlung eines Lenkmoments,

Fig. 2

ein Blockschaltbild mit einer Übersicht über ein Regelsystem einer Überlagerungslenkung zur Ermittlung eines Lenkmoments,

Fig. 3

eine Ausgestaltung des in den Figuren 1 und 2 dargestellten Reglers mit den Regleranteilen die die Lenkung betreffen,

Fig. 4

eine Ausführungsform eines Blocks des in dem in der Figur 2 dargestellten Blockschaltbildes, der ein Lenkmoment ermittelt,

Fig. 5

eine erste Ausführungsform eines Blocks des in dem in der Figur 2 dargestellten Blockschaltbildes zum Ermitteln eines Zusatz-Lenkwinkels,

Fig. 6

eine Ausführungsform eines Blocks zur Störgrößenaufschaltung für den Lenkmomentregler,

Fig. 7

eine Darstellung der Bezugsgrößen an einem Fahrzeug mit den Modellen für die Ermittlung der Schräglauf- und dem Schwimmwinkel,

Fig. 8

eine Kennlinie des Kraftschlussbeiwertes in Querrichtung.

From the figures shows

Fig. 1

a block diagram with an overview of a control system of an electric power steering system for determining a steering torque,

Fig. 2

a block diagram with an overview of a control system of a superposition steering for determining a steering torque,

Fig. 3

an embodiment of the in the Figures 1 and 2 displayed controller with the controller parts that affect the steering,

Fig. 4

an embodiment of a block of the in the in the FIG. 2 illustrated block diagram which determines a steering torque,

Fig. 5

a first embodiment of a block of the in the FIG. 2 illustrated block diagram for determining an additional steering angle,

Fig. 6

an embodiment of a block for feedforward control for the steering torque controller,

Fig. 7

a representation of the reference quantities on a vehicle with the models for the determination of the slip angle and the slip angle,

Fig. 8

a characteristic of the coefficient of adhesion in the transverse direction.

It is assumed that a two-axle, vierrädrigen motor vehicle with steerable wheels at least on a front axle 10 and possibly also on a rear axle 12. In FIG. 1 is a vehicle with a steering actuator shown schematically. A mounted on a steering column 18 steering wheel 20 is connected via a steering gear 22 with the steered wheels 24, 26 of the vehicle. The steering gear 22 is preferably designed as a rack and pinion steering, which has a pinion not shown rotatably connected to the steering column. At the steering column, a torque sensor 14 is arranged, which determines the driver's steering request using a manual steering torque M _H. An electric EPS servo motor 16 (EPS = Electric Power Steering) applied in conventional operation, the steering line with an additional steering torque M _DSR , which amplifies the steering torque M _H applied by the driver.

To set an additional steering torque request _DSR (DSR = Driver Steering Recommendation) for driver assistance, the electric power steering is used, which is controlled by a GCC controller 28 (GCC = Global Chassis Control), for example via an interface to the CAN bus of the vehicle. The controller 28 are thereby set by the driver steering wheel angle δ _L and Hinterachslenkwinkel δ _H , which are arranged with arranged on the steering column 18 and the rear axle 12 steering angle sensors 30, 32 and provided by the torque sensor 14 manual steering torque M _H as input variables available , Furthermore, the controller 28 receives additional sizes of vehicle dynamics controllers and / or driving assistance controllers, as described in more detail in the applications mentioned. The controller 28 determines based on the information provided the additional steering torque M _DSR . The EPS servo motor 16 serves as an actuator, which introduces the steering torque M _DSR (DSR = Driver Steering Recommendation) in correlation with the manual steering torque M _H via the gear 34 in the steering line. Furthermore, the controller 28 calculates a Hinterachslenkwinkel δ _{H, soll} , which is transmitted via a Hinterachslenkeinheit 36 to the rear axle.

Similarly, however, the invention may also be used in vehicles having other steering systems, such as steering systems with external moment interface hydraulic power steering (e.g., APS, Active Power Steering) or a separate torque controller (e.g., IPAS, Intelligent Power Assisted Steering).

FIG. 2 shows a power steering with two steering actuators. The same components or the same blocks have the same reference numerals. At the steering column 18 is in addition to the training of the FIG. 1 a superposition gear 40 is arranged. The superposition gear is usually designed as a planetary gear and divides the steering column into two sections 18a and 18b. The steering wheel angle δ _L measured by the steering wheel angle sensor 32 can be superimposed by means of the superposition gear 40, a further steering angle. The cumulative steering angle δ _V is measured by the steering angle sensor 42 disposed on the portion 18b of the steering column. The superposition steering system 40 is driven by a steering wheel motor 44. The steering wheel motor 44 is controlled by the controller 28 whose reference variable _is the correction steering wheel angle Δδ _soll . To this end, the controller 28 is provided with the steering angle δ _V measured by the front axle steering angle sensor 42. As well as in FIG. 1 described control system receives the controller 28 more Quantities from vehicle dynamics regulations and / or driver assistance regulations.

ein, wobei i_L die Lenkübersetzung ist. Die Lenkübersetzung ist konstant oder kann im Falle einer Überlagerungslenkung auch von weiteren Größen, wie z.B. der Fahrzeuggeschwindigkeit, abhängen. Im Falle einer Lenkwinkelregelung wird der Radlenkwinkel der Vorderachse direkt gemessen.In the additional steering torque M _{DSR of} the wheel steering angle of the front axle δ _V goes to the relationship $${\mathrm{\delta }}_V=\frac{{\mathrm{\delta }}_L}{i_L}$$

a, where i _{L is} the steering ratio. The steering ratio is constant or, in the case of superposition steering, may also depend on other variables, such as vehicle speed. In the case of a steering angle control, the wheel steering angle of the front axle is measured directly.

In the in the Figures 1 and 2 Servomotors 16 shown is preferably provided that the servomotor in the sense of an "intelligent factor" receive a target steering torque from the GCC controller and adjusts this independently. The current manual steering torques M _H are detected by the torque sensor 14 and reported back to the GCC controller 28. The torque sensor 14 is optional, an IPAS does not include a torque sensor. The presence of rear axle steering is not mandatory for the procedure. The other versions, however, assume that the vehicle is equipped with a rear axle steering unit (eg ARK, Active Rear Axle Kinematics). The method for calculating the maximum steering angle is also suitable for pure superposition steering according to FIG. 2 to adjust this value regardless of the driver's default.

a_x

Längsbeschleunigung, gemessen mit einem Längsbeschleunigungssensor oder geschätzt aus Raddrehzahlsignalen

p_B

Bremsdruck, gemessen mit einem Drucksensor (1x Fahrer) oder an den Radbremsen der jeweiligen Rad 24, 26 oder in einem Modell für die vier Radbremsen der Räder 24,26 geschätzt

dψ / dt

Gierrate

a_y

Querbeschleunigung

v_x

Fahrzeuggeschwindigkeit, geschätzt aus Raddrehzahlsignalen

δ _L

Lenkradwinkel

δ _V

Radlenkwinkel Vorderachse

δ _H

Radlenkwinkel Hinterachse.

In Fig. 3 the components and interfaces of the GCC controller 28 are shown. Shown are only the parts that affect the steering. Not shown are controller components for other actuators, such as brake, engine, stabilizer, etc. The steering angle controller 50 and the steering torque controller 52 are either alternatively available or for steering systems as in Fig. 2 represented jointly present. The steering angle controller 50 generates steering angle setpoints Δδ _soll , δ _{H, soll} for the front axle 10 and the rear axle 12. The steering torque controller 52 generates the additional steering torque M _DSR , which represents a driver feedback recommendation (DSR, Driver Steering Recommendation) haptic feedback for the driver. As input variables, the steering angle controller 50 and the steering torque controller 52 are provided with the following variables:

a _x

Longitudinal acceleration measured with a longitudinal acceleration sensor or estimated from wheel speed signals

p _B

Brake pressure, measured with a pressure sensor (1x driver) or estimated at the wheel brakes of the respective wheel 24, 26 or in a model for the four wheel brakes of the wheels 24,26

dψ / dt

yaw rate

a _y

lateral acceleration

v _x

Vehicle speed estimated from wheel speed signals

δ _L

steering wheel angle

δ _V

Wheel steering angle front axle

δ _H

Wheel steering angle rear axle.

In addition, the steering torque regulator 52 is still supplied with the driver's manual torque M _H determined by the torque sensor 14 as an input variable. If the steering angle controller 50 is also present, the steering torque controller 52 is additionally supplied with the desired wheel steering angle change Δδ _soll as the input variable.

An exemplary embodiment of the steering torque controller 52 in understeer situations is in Fig. 4 shown. An embodiment for the steering angle controller 50 in understeer situations shows Fig. 5 ,

Both controllers 50, 52 have the following basic structure of the steering column control system for determining the steering torque request M _DSR or the steering angle request Δδ _soll . Driving situations in which there is an understeering driving condition of the vehicle are detected in blocks 60 and 62. In doing so, they resort in particular to information provided by a driving dynamics controller. The driving state controller may be, for example, an ESP and / or an ABS system. A detection of critical driving situations in which the vehicle is understeering is preferably carried out in block 60 on the basis of an ESP understeer recognition. In block 62, an understeer of the vehicle is alternatively detected by means of a slip angle understeer identifier.

The detection of an understeer situation occurs in both controllers 50, 52 here for two alternatives. An understeer control, which is extended by the rear steering part in the ESP, uses the linear stationary single-track model $$\dot{\mathrm{\psi }}=\frac{v_x}{l+\mathit{EC}{v}_x^2}\left({\mathrm{\delta }}_V-{\mathrm{\delta }}_H\right)\mathrm{,}$$

The model (3.1) provides a reference for the Vorderachslenkwinkel in the form $${\mathrm{\delta }}_{V.\mathit{ref}}=\frac{l}{v_x}\dot{\mathrm{\psi }}+{\mathit{EGa}}_y+{\mathrm{\delta }}_H\mathrm{,}$$

einen vorgegebenen Schwellenwert S _δ überschreitet. Die zweite Möglichkeit zur Untersteuererkennung basiert auf dem Schräglaufwinkel an der Vorderachse $${\mathrm{\alpha }}_V=-{\mathrm{\delta }}_V+\mathrm{\beta }+\frac{l_V}{v_x}\dot{\mathrm{\psi }}\mathrm{.}$$

und dem Schräglaufwinkel an der Hinterachse, vgl. Fig. 7 $${\mathrm{\alpha }}_H=-{\mathrm{\delta }}_H+\mathrm{\beta }-\frac{l_H}{v_x}\dot{\mathrm{\psi }}\mathrm{.}$$

An understeer is detected when the difference $$\left|{\mathrm{\delta }}_V\right|-\left|{\mathrm{\delta }}_{V.\mathit{ref}}\right|>S_{\mathrm{\delta }}$$

exceeds a predetermined threshold S _δ . The second option for understeer detection is based on the slip angle at the front axle $${\mathrm{\alpha }}_V=-{\mathrm{\delta }}_V+\mathrm{\beta }+\frac{l_V}{v_x}\dot{\mathrm{\psi }}\mathrm{,}$$

and the slip angle at the rear axle, cf. Fig. 7 $${\mathrm{\alpha }}_H=-{\mathrm{\delta }}_H+\mathrm{\beta }-\frac{l_H}{v_x}\dot{\mathrm{\psi }}\mathrm{,}$$

For detection, not the individual slip angles are needed, only the difference $$\mathrm{\Delta }\mathrm{\alpha }={\mathrm{\alpha }}_V-{\mathrm{\alpha }}_H={\mathrm{\delta }}_H-{\mathrm{\delta }}_{\mathit{V}}+\frac{l}{v_x}\dot{\mathrm{\psi }}\mathrm{,}$$

Depending on a threshold for skew angle difference (3.6) and the sign of the yaw rate, an understeer is detected, if applicable $$\begin{array}{c}d\mathbf{\psi }/\mathit{dt}>0\mathbf{and}\Delta \mathrm{\alpha }<-S_{\mathrm{\alpha }}\hfill \\ \mathbf{or}\\ d\mathbf{\psi }/\mathit{dt}<0\mathbf{and}\Delta \mathrm{\alpha }>S_{\mathrm{\alpha }}\hfill \end{array}$$

The threshold S _α is between 2 and 10 degrees, preferably at 5 degrees. If an understeer situation is detected in one of the blocks 60, 62 on the basis of exceeding the threshold values S _δ or S _α , the sub-control flag 64, which represents the output signal of the block 60 or 62, is set to the value 1. The understeer flag is reset from the value 1 to the value 0 when the above conditions are no longer met. Preferably, however, smaller threshold values are used as a basis, so that the control is calmed by a hysteresis. The threshold values may depend on other parameters of the driving dynamics, such as the vehicle speed v _x or the road friction coefficient μ. As the driving speed decreases, the threshold values are increased and correspondingly reduced as the road friction coefficient decreases.

The blocks 60, 62 are connected via an OR gate 66 to an activation logic 68 for activating the control system. In the activation logic 68, the wheel steering angle of the front axle δ _V , the limited wheel steering angle of the front axle δ _{V , lim} , whose determination will be described later and the understeer flag 64 as an input signal.

wird die Lenkmomentregelung 52 aktiviert, indem ein Untersteuer-Aktiv-Flag, welches das Ausgangssignal der Aktivierungslogik 68 darstellt, auf den Wert 1 gesetzt.In fulfilling the conditions $$\begin{array}{c}{\mathbf{\delta }}_{V.\lim }<\left|{\mathbf{\delta }}_V\right|\\ \mathbf{and}\mathrm{under\; control}-\mathrm{Flag}=1\end{array}$$

For example, the steering torque control 52 is activated by setting an understeer active flag representing the output of the activation logic 68 to a value of 1.

oder nach Beendigungsbedingungen, die eine Beendigung nach Ablauf einer vorgegebenen Zeit vorsehen.The steering torque control 52 is terminated and the output signal under control active flag of the activation logic 68 is set to 0 if the following conditions are met: $$\begin{array}{c}{\mathbf{\delta }}_{\mathrm{V}.\lim }>\left|{\mathbf{\delta }}_{\mathrm{V}}\right|\\ \mathbf{or}\mathrm{under\; control}-\mathrm{Flag}=0\end{array}$$

or after termination conditions that provide for termination after expiration of a predetermined time.

Each of the controllers 50, 52 contains a steering angle limitation determination unit 70, which supplies the yaw rate dψ / dt , the longitudinal acceleration a _x , the lateral acceleration a _y , the vehicle speed v _x as input variables.

The steering angle limit serves to determine a limitation of the wheel steering angle at the front axle. For this purpose, the following polynomial model of lateral force is used $$F_y=\{\begin{array}{cc}-\left(C_{a0}{F}_z\mathrm{\alpha }-\mathrm{sign}\left(\mathrm{\alpha }\right)\frac{C_{\mathrm{\alpha }0}^2{F}_z^2}{4{\mathrm{\mu }}_0{F}_z}{\mathrm{\alpha }}^2\right)\sqrt{1-{\left(\frac{F_x}{{\mathrm{\mu }}_0{F}_z}\right)}^2}& .\left|\mathrm{\alpha }\right|<\frac{2{\mathrm{\mu }}_0}{C_{\mathrm{\alpha }0}}\\ -\mathrm{sign}\left(\mathrm{\alpha }\right){\mathrm{\mu }}_0{F}_z\sqrt{1-{\left(\frac{F_x}{{\mathrm{\mu }}_0{F}_z}\right)}^2}& .\left|\mathrm{\alpha }\right|\ge \frac{2{\mathrm{\mu }}_0}{C_{\mathrm{\alpha }0}}\end{array}$$

sein Maximum. Mit (3.8) kann aus (3.4) der zum maximalen Kraftschlussbeiwert korrespondierende Lenkwinkel bestimmt werden als $${\mathrm{\delta }}_V{|}_{{\mathrm{\alpha }}_V={\mathrm{\alpha }}_{\lim }}=\frac{l_V}{v_x}\dot{\mathrm{\psi }}-\mathrm{sign}\left({\mathrm{\alpha }}_V\right){\mathrm{\mu }}_0\frac{2}{C_{\mathrm{\alpha }0}}+\mathrm{\beta }\mathrm{.}$$

If one relates the side force F _y to the contact force F _z , then one obtains from the model (3.7) the in Fig. 8 illustrated characteristic of the coefficient of adhesion in the transverse direction. The coefficient of adhesion reaches at the slip angle $${\mathrm{\alpha }}_{\lim }\mathrm{=}{\mathrm{\mu }}_{\mathrm{0}}\frac{\mathrm{2}}{{\mathrm{C}}_{\mathrm{\alpha }\mathrm{0}}}$$

his maximum. With (3.8), the steering angle corresponding to the maximum adhesion coefficient can be determined from (3.4) as $${\mathrm{\delta }}_V{|}_{{\mathrm{\alpha }}_V={\mathrm{\alpha }}_{\lim }}=\frac{l_V}{v_x}\dot{\mathrm{\psi }}-\mathrm{sign}\left({\mathrm{\alpha }}_V\right){\mathrm{\mu }}_0\frac{2}{C_{\mathrm{\alpha }0}}+\mathrm{\beta }\mathrm{,}$$

The road friction coefficient μ ₀ and the slip angle β can not be measured economically in the vehicle. For the slip angle is approximately at understeer $$\mathrm{\beta }\mathrm{\approx }\mathrm{0th}$$

mit der Kraftschlussausnutzung für die Vorderachse $${\mathrm{\mu }}_{\mathit{VA}}=\frac{\sqrt{{\left(a_x-l_V{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_V\ddot{\mathrm{\psi }}\right)}^2}}{g},$$

der Kraftschlussausnutzung im Fahrzeugschwerpunkt $${\mathrm{\mu }}_{\mathit{CoG}}=\frac{\sqrt{a_x^2+a_y^2}}{g}$$

und der Kraftschlussausnutzung an der Hinterachse $${\mathrm{\mu }}_{\mathit{HA}}=\frac{\sqrt{{\left(a_x+l_H{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_H\ddot{\mathrm{\psi }}\right)}^2}}{g}\mathrm{.}$$

An estimate of the road friction coefficient based on vehicle center of gravity (CoG Center of Gravity) or front and rear axle accelerations will result $${\hat{\mathrm{\mu }}}_0=\mathrm{Max}\left({\mathrm{\mu }}_{\mathit{VA}},{\mathrm{\mu }}_{\mathit{CoG}},{\mathrm{\mu }}_{\mathit{HA}}\right).$$

with the adhesion utilization for the front axle $${\mathrm{\mu }}_{\mathit{VA}}=\frac{\sqrt{{\left(a_x-l_V{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_V\ddot{\mathrm{\psi }}\right)}^2}}{G}.$$

the adhesion utilization in the vehicle's center of gravity $${\mathrm{\mu }}_{\mathit{CoG}}=\frac{\sqrt{a_x^2+a_y^2}}{G}$$

and the adhesion utilization at the rear axle $${\mathrm{\mu }}_{\mathit{HA}}=\frac{\sqrt{{\left(a_x+l_H{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_H\ddot{\mathrm{\psi }}\right)}^2}}{G}\mathrm{,}$$

ergibt sich die gesuchte Begrenzung für den Radlenkwinkel an der Vorderachse betragsmäßig zu $${\mathrm{\delta }}_{V,\mathit{lim}}=\frac{l_V}{v_x}\left|\dot{\mathrm{\psi }}\right|+\frac{2}{C_{\mathrm{\alpha }0}}{\hat{\mathrm{\mu }}}_0$$

Taking into account the relationship between the signs of slip angle and lateral acceleration $$\mathrm{sign}\left({\mathrm{\alpha }}_{\lim }\right)=-\mathrm{sign}\left(a_y\right)$$

the desired limitation for the wheel steering angle at the front axle results in terms of amount $${\mathrm{\delta }}_{V.\mathit{lim}}=\frac{l_V}{v_x}\left|\dot{\mathrm{\psi }}\right|+\frac{2}{C_{\mathrm{\alpha }0}}{\hat{\mathrm{\mu }}}_0$$

The parameter C _α0 can be dependent on the road _{friction coefficient} and must be applied during the driving test.

The assumption for the calculation of the steering angle limitation was a small slip angle according to (3.10). It must be assumed that the vehicle turns increasingly and thus increases the slip angle. Therefore, the limitation may only be made for a certain time (preferably 4s). In the case of a haptic system, the increase in the steering torque must then be withdrawn. In a superposition steering the additional steering angle is reduced again after this time.

The limited wheel steering angle δ _{V , lim} calculated in accordance with 3.16 is provided to the activation logic 68, which is based on the conditions described above, the steering torque control 52 is activated or terminated.

For Lenkmoment- or steering angle control of the current wheel steering angle δ _{V of} the front axle with reversed sign passes through a transmission member 72 with dead zone. The deadband is defined between the positive and negative values of the current wheel steering angle limit (3.16). The output of the dead zone transfer member 72 is the control deviation $$e_{\mathrm{\delta }}=\{\begin{array}{ccc}-{\mathrm{\delta }}_V-{\mathrm{\delta }}_{V.\lim }& \mathrm{For}& -{\mathrm{\delta }}_V\ge {\mathrm{\delta }}_{V.\lim }\hfill \\ 0& \mathrm{For}& -{\mathrm{\delta }}_V>-{\mathrm{\delta }}_{V.\lim }\hfill \\ -{\mathrm{\delta }}_V+{\mathrm{\delta }}_{V.\lim }& \mathrm{otherwise}& \end{array}$$

Within the dead zone, the control deviation is zero, outside it is the value of the wheel steering angle δ _V which is reduced by the limit. The control deviation e _δ is fed to a controller 74. The controller 74 may be implemented as a simple P-controller or as a dynamic controller. If a superposition steering ( Figure 5 ), the nominal wheel steering angle change Δδ _{soll can} also _be used as pre-control in the sense of a feedforward control for the steering torque control of the steering torque controller 52, as shown in FIG FIG. 6 , The _{controller output variable} u _M or u _Δδ is optionally limited in its height and its rise by a limiting member 76. The parameters of the controller 74 and the limiting member 76 are to be set depending on the vehicle. A limitation taking into account the current driver's manual torque is also possible.

LIST OF REFERENCE NUMBERS

a _x

Longitudinal acceleration, possibly estimated from wheel speed signals

p _B

Brake pressure, 1x driver, 4x wheels possibly appreciated

dψ / dt, ψ̇

yaw rate

a _y

lateral acceleration

v _x

Vehicle speed estimated from wheel speed signals

δ _L

steering wheel angle

δ _V

Wheel steering angle front axle

δ _{V, lim}

Limitation wheel steering angle front axle

δ _{V, ref}

Reference value wheel steering angle front axle

Δδ _should

Target wheel steering angle change front axle

δ _H

Wheel steering angle rear axle

δ _{H, shall}

Target wheel steering angle rear axle

M _H

Driver's hand on the steering wheel

M _DSR

Target steering torque

β

float angle

α

Slip angle

α _lim

Slip angle at side force maximum or adhesion coefficient maximum

Δα

Slip angle difference front axle - rear axle α _V -α _H

F _y

lateral force

F _z

contact force

μ ₀

road friction coefficient

μ _y

Coefficient of adhesion in the transverse direction

μ _{y, max}

Kraftschlussbeiwertmaximum

C _α0

Initial slope coefficient of adhesion slip angle curve

EC

self-steering gradient

I

wheelbase

I _V

Distance center of gravity - front axle

I _H

Distance center of gravity - rear axle

e _δ

deviation

10

Front

12

rear axle

14

torque sensor

16

servomotor

18

steering column

20

steering wheel

22

steering gear

24

bikes

26

bikes

28

Gcc controller

30

Steering angle sensor

32

Steering angle sensor

34

transmission

36

Hinterachslenkeinheit

40

Superposition gear

42

Steering angle sensor

44

steering wheel motor

50

Steering angle controller

52

Steering torque controller

60

block

62

block

64

Under control flag

66

OR gate

68

activation logic

70

determining unit

72

transmission member

74

regulator

76

limiting member

Claims (7)

1. Method for determining an optimum steering angle in driving situations of a vehicle, a first portion which represents the adhesion coefficient in the lateral direction being taken into account in the determination, a second portion which represents a kinematic portion being taken into account, and a third portion which represents the attitude angle being taken into account, and the steering angle (δ _v,lim) being determined by adding the portion of the adhesion coefficient, the kinematic portion and the attitude angle, characterized in that during understeering situations of the vehicle the attitude angle is estimated according to the relationship β ≈ 0.
2. Method according to Claim 1, characterized in that the first portion takes into account a coefficient of friction of the underlying surface
   µ̂₀ = max(µ _VA , µ _CoG , µ _HA ) according to at least one of the following relationships,
   utilizing the adhesion for the front axle $${\mathrm{\mu }}_{\mathit{VA}}=\frac{\sqrt{{\left(a_x-l_V{\mathrm{\psi }}^2\right)}^2+{\left(a_y-l_V\mathrm{\psi }\right)}^2}}{g},$$

   

   
   or
   utilizing the adhesion at the center of gravity of the vehicle ${\mathrm{\mu }}_{\mathit{CoG}}=\frac{\sqrt{a_x^2+a_y^2}}{g},$

   

   or
   utilizing the adhesion at the rear axle $${\mathrm{\mu }}_{\mathit{HA}}=\frac{\sqrt{{\left(a_x+l_H{\dot{\mathrm{\psi }}}^2\right)}^2+{\left(a_y-l_H\ddot{\mathrm{\psi }}\right)}^2}}{g}\mathrm{.}$$

   
3. Method according to either of Claims 1 and 2, characterized in that the steering angle is determined in terms of absolute value according to the relationship $${\mathrm{\delta }}_{V,\mathit{lim}}=\frac{l_V}{v_x}\left|\dot{\mathrm{\psi }}\right|+\frac{2}{C_{\mathrm{\alpha }0}}{\hat{\mathrm{\mu }}}_0\mathrm{.}$$

   
4. Method according to one of Claims 1 to 3, characterized in that the steering angle δ_v,lim or a steering angle δ_v,lim which is multiplied by a factor k is used as a setpoint value for a steering angle control means or a steering torque control means.
5. Method according to either of Claims 1 and 2, characterized in that a steering torque control means is activated according to the relationship $$\begin{array}{c}{\mathbf{\delta }}_{v,\lim }<\left|{\mathbf{\delta }}_v\right|\end{array}\mathrm{.}$$

   
6. Method according to one of the preceding claims, characterized in that a steering torque control means is deactivated according to the relationship $$\begin{array}{c}{\mathbf{\delta }}_{v,\lim }<\left|{\mathbf{\delta }}_v\right|\end{array}\mathrm{.}$$

   
7. Device for determining an optimum steering angle in understeer situations of a vehicle, characterized by a determining unit (70) for determining a stabilizing steering angle taking into account a model-based portion of the adhesion coefficient, a model-based kinematic portion and an attitude angle

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